The present invention relates to a sensor for detecting frost deposits, particularly on evaporators in refrigeration plants and the like.
In refrigeration plants, such as cold stores and the like as a result of the cooling, in the case of supersaturated steam in the cooling chamber sublimation of the water vapor takes place on the cold fins or ribs or gills of the evaporator, so that frost is formed thereon. In the case of frost formation, part of the heat necessary for evaporating the cooling fluid in the evaporator does not come from the environment of the fins, so that they are further cooled, i.e. their temperature is decreased and instead use is made of the solidification or sublimation heat given off during frost formation. In the case of frost formation, there is also an increase of the fixed surface at which sublimation of the water vapour can take place. There is an overall decrease in the efficiency of the refrigeration plant, so that finally it is necessary to defrost the frost formed on the evaporator which once again calls for an energy supply. Defrosting takes place by incorporated electrical tubular heaters, which also pass through the fins, e.g. between the cold guide pipes passing through the evaporator fins. These electrical tubular heaters are called defrosting heaters. As such defrosting processes must be completed within a short time, because only the frost on the evaporator fins is to be removed by the heat supply, whereas, the cooling chamber must not be heated, a relatively large electrical capacity must be installed.
At present, evaporator defrosting takes place in different ways. Usually defrosting takes place by time synchronization, i.e. a time switch is provided, which switches in the defrosting plant at preselected time intervals, so that the frost is removed from the evaporator fins. With this time switching method, it is not possible to take account of the thickness of the frost or ice deposit, i.e. it is not possible in the optimum manner to defrost a frost layer of a certain, not excessive thickness and instead defrosting either takes place too early when there is still no frost deposit, or too late when the evaporator is already significantly frosted up. Attempts have been made to improve this in that switching in is not time-controlled and instead takes place via thermistors (U.S. Pat. No. 4,305,259). However, here again the heating period remains fixed. Due to its limited thermal conductivity coefficient, it is not possible to determine frost formation with its low density.
Another method takes account of the fact that the through-flow of air decreases with increasing frost deposits on the evaporator fins. Thus, the differential pressure is determined by measurement before and after evaporation. However, faulty switching can be caused by contamination and it is not always adequately accurately possible to detect the frost thickness.
In addition, a photo-optical method is known, in which a light beam is directed onto a receiver, so that when the light beam is interrupted by frost formation a signal is transmitted, which switches on the defrosting heater. Here again faults can occur due to contamination, as well as frost and ice formation on the transmitter and receiver, so that the defrosting plant is almost permanently switched on, although this is not required by the actual frost formation on the fins. Thermostatic control is also known, but is not suitable for detecting frost formation. Another measuring principle based on conductivity changes provides electrodes as test probes. Snow or ice or a medium arranged in a casing is initially melted and liquid between the electrodes once again switches on a defrosting plant (DE-OS Nos. 21 51 876, 31 09 366 and DE-AS No. 25 14 489). A method in which the conductivity between adjacent electrodes is measured directly without heating is based on the same principle (DE-OS No. 33 10 327). In addition, DE-OS No. 21 04 302 discloses producing heat by current, via a metal part acting as an electrical resistor, and to the metal part are soldered constant thermocouple wires to constitute a thermocouple, which through delays in the heating of the metal part can establish ice formation thereon.
In the known methods, unnecessary switch on/off signals for the defrosting heater are transmitted at the incorrect time, so that excessive energy and consequently great expense are expended.
The problem of the invention is to provide a sensor which, while avoiding the aforementioned disadvantages, very accurately detects frost formation and as a function thereof supplies a signal for switching on the defrosting plant as a function of the desired setting quantity.
According to the invention the above problem is solved with a sensor of the aforementioned type in that between a heat source and a heat sensor, which are spaced from one another, is provided a thermal connection of a material, whose thermal conductivity coefficient is of the same order of magnitude as the thermal conductivity coefficient of frost. Although the means according to the invention is particularly intended for detecting frost formation between the evaporator fins of a refrigeration plant, it can also be used for detecting snow and ice at switch points, in gutters and the like.
The invention is based on the fact that the heat produced by the heat source passes to the heat sensor, where it is detected. It is based on the idea that through frost formation an adequate heat proportion is removed from the aforementioned heat flow and is of the same order of magnitude as the heat quantity passing from the heat source to the heat sensor, so that much less heat passes to the latter than when there is no frost and the difference in the heat quantity in the absence of frost and in the presence of frost is of the same order of magnitude as the absolute heat quantity. It is important in this connection that the thermal conductivity coefficient of the material connecting the heat source and the heat sensor is of the same order of magnitude as the thermal conductivity coefficient of frost. If this connecting material has a much higher thermal conductivity coefficient, this would mean that even in the case of frost formation which, due to the "porosity" of frost has a very low thermal conductivity coefficient, virtually no heat would be removed and instead most of the heat produced would pass from the heat source to the sensor and the removed heat quantity, which is e.g. in the permille or lower percent range of the heat passing to the sensor and consequently within the error limit, e.g. through different cooling temperatures, electronic faults, etc. so that no usable signal is produced, i.e. despite frost formation the heat sensor would not transmit a signal for switching on the defrosting heater.
However, as a result of the sensor construction according to the invention, it is possible to obtain a usable electrical signal for switching the defrosting plant in a sufficiently accurate manner as a result of the adequately different heat flow and the resulting differing influencing of the sensor in the case of frost and ice formation.
According to a preferred development, the heat source and heat sensor are arranged in a common casing aud the latter forms the connection with a thermal conductivity coefficient of the same order of magnitude as that of frost. Materials for the same can be selected from the relevant tables (e.g. according to Dubbel) due to the conditions involved with the solution according to the invention. A preferred sleeve or casing material is e.g. silicone rubber provided with additives or admixtures, such as carbon black, because this material has the necessary stability and elasticity, apart from the thermal conductivity coefficient which can be set through its density and in particular the admixture of carbon black.
In a preferred manner, the heat source has a PTC resistor.
Thus, even in the case of varying environmental conditions, the heat quantity reaching the heat sensor fluctuates very little, assuming the same degree of frosting. In this case, the sensor can be constructed on the basis of a sleeve-like resistant heater according to German Pat. No. 29 48 592 and in addition to the PTC resistor as the heat source in the sleeve is provided a heat sensor and it must be ensured that if there is contacting of the PTC resistor via metallic contact plates, their dimensions must be adapted to those of the PTC resistor and must never extend up to the heat sensor, so that no heat bridge with a high thermal conductivity coefficient is formed.
According to a further development, the heat sensor is constructed as a NTC-resistor, or is constructed as a semiconductor and in particular silicon heat sensor, which has a good, accurate measuring range between -50.degree. C. and 0.degree. C.